Computer programs and methods for generating 1-bit image data from multiple-bit image data

ABSTRACT

Methods of printing a color image having more than one color comprise receiving multiple-bit image data comprising multiple-bit pixel values, deriving 1-bit image data comprising first and second sets of 1-bit image data, and printing from the 1-bit image data the color image. Methods of generating 1-bit image data for a color image having more than one color comprise receiving multiple-bit image data comprising multiple-bit pixel values, and electronically deriving a first set and a second set of 1-bit image data.

RELATED APPLICATIONS

The present application is a continuation of, and claims priority under35 U.S.C. §120 to, U.S. patent application Ser. No. 13/898,676, whichwas filed on May 21, 2013, issued on Nov. 29, 2016 as U.S. Pat. No.9,508,031, and which claims priority to U.S. Provisional PatentApplication No. 61/650,270, which was filed on May 22, 2012. Thecomplete disclosures of the above-identified patent and patentapplication are incorporated herein by reference.

FIELD

The present disclosure relates to computer programs and methods forgenerating 1-bit image data from multiple-bit image data.

BACKGROUND

It is well-known to convert multiple-bit image data into 1-bit imagedata comprising “on” and “off” pixel values to enable an image havingvarying intensity levels to be printed so that the printed image isconstituted by a plurality of dots, the number density or size of whichvaries in order to represent the varying intensity levels.

Such programs fall into two broad categories. The first category carriesout so-called “amplitude modulated” (AM) screening to generate 1-bitimage data that, when printed, produce an image that is constituted by aplurality of sizes of dots regularly arranged throughout the image. Thesecond category of programs carries out so-called “frequency modulated”(FM) screening to generate 1-bit image data that, when printed, producean image that is constituted by a plurality of number densities of dots,substantially all of the dots typically having the same size.

FM screening is widely considered to provide the potential for higherquality image reproduction than AM screening, but has not been widelyadopted. One of the issues affecting FM (also known as stochastic)screening has been the graininess (noise and patterning) of the printedresult. Various proposals have been made to reduce or eliminate noise inFM screened images, but these techniques have only been developed inrelation to monochrome printing. The techniques do not eliminate noisethat can arise in FM colour images or colour errors which can occur(especially at mid intensity levels) as a result of mis-registration ofthe arrays of different coloured dots laid down in the printing process.

SUMMARY

Methods of printing colour images, methods of generating 1-bit imagedata for colour images, printing apparatus, and electronic dataprocessing apparatus are disclosed.

Methods of printing colour images having more than one colour andderived from multiple bit image data comprise the steps of (a) receivingmultiple-bit image data comprising multiple-bit pixel values; (b)deriving from said multiple-bit pixel values 1-bit image data comprisinga first set and a second set of 1-bit image data comprising “on” and“off” pixel values, each set corresponding to a respective componentcolour of the colour image; and (c) printing from the 1-bit image datathe colour image comprising corresponding first and second sets of dots.Methods of generating 1-bit image data for colour images having morethan one colour comprise the steps of: (a) receiving multiple-bit imagedata comprising multiple-bit pixel values; and (b) using electronic dataprocessing apparatus, electronically deriving from said multiple-bitpixel values a first set and a second set of 1-bit image data comprising“on” and “off” pixel values which produce when printed a first andsecond set of dots.

In such methods, the dots of each set are of a respective one of thecomponent colours. Each of all of the dots of one of the colours of dotsare constrained to be within a respective site selected from a first setof predetermined possible sites. Each of all of the dots of the othercolour of dots are constrained to be within a respective site selectedfrom a second set of predetermined possible sites. The possible sites ofthe first and second sets of possible sites overlap so that individualdots of one colour of dots can overlap dots of the other colour of dots.The possible sites of one set are of different dimensions from the sitesof the other set so that individual dots of one colour cannot fullycover individual dots of the other colour. The possible sites have thesame shape and size but different dimensions by virtue of the sites ofthe first set of possible sites having a different orientation from thesites of the second set of possible sites.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B illustrate two possible ways in which dots may bedeposited in a printing process using FM screening to determine theposition of printed dots of four component colours (in the CMYK printingsystem) on a substrate.

FIG. 2 is a screen capture of a mix of colours obtained by an FMscreening process (when applied to two colour components), illustratingresultant variations in perceived intensity.

FIG. 3 shows a grid forming a small part of an output image created by aprocess in accordance with the present disclosure, the image in thisexample being constituted by two component colours, magenta and cyan.

FIG. 4 is a corresponding view of the grid of FIG. 3, marked-up to showthe possible sites for the cyan dots.

FIG. 5 is a corresponding view of the grid of FIG. 3, marked-up tohighlight the possible sites for the magenta dots.

FIG. 6 is a corresponding view of the grid of FIG. 3, marked-up withreduced size dots.

FIG. 7 is a screen capture corresponding to FIG. 2, illustrating how theprocess in accordance with the present disclosure achieves improvementin the consistency of the overlap between dots.

FIG. 8 is a schematic representation of a portion of an array of 8-bitpixel values that constitute multiple-bit image data.

FIG. 9 is a schematic representation of a grid of possible sites fordots.

FIG. 10 is a schematic representation of a printing apparatus inaccordance with the present disclosure.

DESCRIPTION

The present disclosure relates to computer programs and methods forgenerating 1-bit image data from multiple-bit image data. In someembodiments, computer programs and methods for generating 1-bit imagedata that, when printed, produces a colour image constituted by dots.The present disclosure also relates to methods for printing colourimages derived from multiple-bit image data, and to printing apparatusesfor printing such images.

According to a first aspect of the present disclosure, there is provideda computer program for executing on a computer system a computer processfor generating 1-bit image data for a colour image, the 1-bit image datacomprising at least two sets of pixel values, each set corresponding toa respective component colour of the image. The computer process mayinclude the steps of (a) receiving multiple-bit image data comprisingmultiple-bit pixel values; and (b) deriving from said multiple-bit pixelvalues a first and second set of 1-bit image data comprising “on” and“off” pixel values. The 1-bit image data may produce, when printed, animage comprising corresponding first and second sets of dots, the dotsof each set being of a respective one of the component colours. Each dotof one of the sets of dots may be contained within a respective siteselected from a first set of possible sites, and each dot of the otherset of dots may be contained within a respective site selected from asecond set of possible sites. The sites of the first and second sets ofpossible sites may overlap so that individual dots from one set of dotscan overlap dots from the other set of dots. The sites of one set are ofdifferent dimensions from the sites of the other set, so that individualdots from one set cannot fully cover individual dots from the other set.

In some embodiments, each set of possible sites is tessellated, and thetwo sets of possible sites cover the same area of the image. Thepossible sites may be of different dimensions by virtue of havingdifferent sizes, shapes or orientations.

In some embodiments, the possible sites from one set are each of a sizeof M1×N1 pixels, whilst those of the second set are of a size M2×N2pixels, where M1 does not equal M2 and N1 does not equal N2. Thus, forexample, the dimensions of each member of one set of possible sitescould be two units high by three units wide (2×3), whilst those of eachpossible site from the other set would be three units high by two unitswide (3×2). Each such unit may correspond to a respective pixel in theoutput image so that the sites of the first set of possible sites areeach defined by two rows and three columns of pixels, the second set ofpossible sites each being defined by three rows and two columns ofpixels. The possible sites could thus be considered to have the sameshape (i.e. rectangular) and size (six pixels) but have differentorientations, one rectangle being angularly displaced by 90° relative tothe other.

Each dot may be co-extensive with its respective site, so that the dothas the same size, shape and orientation as its respective site.Alternatively, the sites may not be fully populated, so that each dot issmaller than its respective site. This can result in the dots of the twosets being of the same size, shape and orientation, but since each dotof a given set is constrained to be within its respective site, the dotscan be so arranged (for example with all the dots of one set being inthe same position in their corresponding sites) so that a significantnumber of individual dots from one set still cannot completely coverdots from the other set.

In known FM screening techniques, for colour images, the dots for eachset would share the same possible sites, so that a dot from one setcould completely cover a dot from the other set or in other cases dotsfrom one set could be interleaved between those from the other set. Dueto the way in which ink filters light, a zone which has a largeproportion of dots covered by other dots will appear lighter than a zonewhich has the same number of dots, but has a greater proportion ofinterleaved dots. By ensuring that the number of instances of dots fromone set completely covering those from another set is at least reduced,some methods according to the present disclosure ameliorate or avoid theproblems caused by the aforesaid variations.

Since many dots will only partially overlap, the effect ofmis-registration on colour is also reduced. This is especially pertinentif an FM screening technique is being used to render an area in a toneconstituted by the two colours when at their mid intensity levels.Although there is a random element to the way in which the standardalgorithms lay down dots, the error diffusion algorithms which are alsoused result in dots for an area of mid tone being presented in achequer-board pattern. Since this can happen for both colour components,the conventional techniques can result in all of the dots from one setbeing completely covered by registering dots from the other set.However, any mis-registration will cause a very noticeable colour errorbecause the coloured inks filter light much less effectively when thedots are completely in register. However, if dots are laid down inaccordance with a program or method of the present disclosure, then thenormal relationship between the dots of different sets will be one inwhich very few dots are fully covered by dots of the other set, so thatany mis-registration will have a less, or no, noticeable effect oncolour.

A particular problem that may arise with printing processes that utiliseFM screening is patterning which can result from isolated pairs ofdiagonally adjacent dots exhibiting greater dot gain than isolated pairsof horizontally or vertically adjacent dots. This is discussed in PCTInternational Publication No. WO 2011/030101 and U.S. Patent ApplicationPublication No. 2012/0218607 (the disclosures of which are herebyincorporated by reference), in which the problem is at least amelioratedby reducing the size of each dot in the output data, so that each dot isno longer co-extensive with its site. However, the dot is still the onlydot allocated to that particular site so that spacing between adjacentdots can be ensured. The reduction in size can be done by omitting thefirst row and/or column of the pixels defining each dot.

It will be appreciated that, in the present case, this reduction maylead to the dots of the two sets being of the same dimensions as eachother, as mentioned above. For example, omitting the first column of a2×3 pixel dot and the first row of a 3×2 pixel dot will produce two 2×2pixel dots.

If the placement of such dots were unconstrained, then it is possiblefor a significant number of dots of one set to be completely covered by(and co-extensive with) the dots of the other set. However, significantamounts of covering can be avoided since each dot is constrained toremain in its site, and the reduced size dots of each set can occupycorresponding portions of their sites.

Thus, where the size of the dots is reduced so that the dots of bothsets are the same size, this may be done in a consistent manner for thedots in each set, so that the dots in each set all occupy the samerespective portion of their sites. For example, if the 3×2 and 2×3 dotsare being reduced in size, this preferably involves omitting the firstrow of all of the 3×2 dots and the first column of all of the 2×3 dots.

It is then possible for some dots to be completely covered by others,but this will only apply to a minority of dots, and at regular intervalswhich occur over the output medium with a high frequency of repetition,and is generally imperceptible to the eye. In the example mentionedabove, a 2×2 dot for one set can be covered completely by a 2×2 dot forthe other set only in one corner of an image cell, the cell being thesmallest area that can accommodate an integral number of possible sitesfrom each set (for example a cell of 6×6 pixels for 2×3 and 3×2 pixelpossible sites). In some embodiments, the width of each site in any oneof the sets may not be a multiple or a factor of the width of each sitein the other set. Thus, the sites are not in phase with each other sothat one possible site cannot be sandwiched (i.e. interleaved) betweentwo adjacent possible sites from the other set. Consequently, a dot fromone set can only ever be concatenated with one non-overlapping dot fromthe other set. When the intensity to be represented by the dots isaround 50% of maximum, this avoids setting up a series of interleaved,concatenated dots from alternating sets, and thus avoids the consequentpatterning which may occur in the final image.

In some embodiments, the pixel data for one of the sets of dotsassociated with possible sites of 2×3 in size or the dots associatedwith possible sites of 3×2 units in size is for the cyan component ofthe image, whilst that for the other set is for the magenta component.

In some embodiments, the image data may include data for a third set ofdots, and associated possible sites, corresponding to the blackcomponent, the size of each site of the set being 4×4 pixels. Thereduced size version of these dots may be 3×4 or 4×3 pixels.

According to a second aspect of the disclosure, there is provided aprinting apparatus comprising a printer, and a computer programmed with,and operable to execute, a program in accordance with the first aspectof the present disclosure. The computer may generate a plurality of setsof 1-bit image data comprising “on” and “off” pixel values, each ofwhich corresponds to a pixel of an output medium that the printer wouldattempt to mark, in the colour associated with that pixel's set whenprinting the 1-bit data if the pixel were on.

Also within the scope of the present disclosure is a method of printinga colour image derived from multiple bit image data. The method mayinclude (a) converting the multiple bit image data into 1-bit image datacomprising two sets of “on” and “off” pixel values, each setcorresponding to a respective component colour of the image, and (b)printing the two sets of data onto an output medium to create two setsof dots, each of which dots is located at a respective site, each of arespective component colour, at positions which may overlap. The sitesfor the dots of one set are of different dimensions from those of theother set, so that the dots may be arranged such that individual dotsfrom one set cannot completely cover, or be completely covered by,individual dots from the other set.

Where appropriate, the various methods disclosed herein may be describedas computer-implemented methods, server-implemented methods, and/orprinter-implemented methods. Moreover, such computer, servers, and/orprinters may be configured to utilize, implement, and/or otherwisefacilitate various methods according to the present disclosure.Computers, servers, and/or printers may include or be configured to readcomputer readable storage, or memory, media suitable for storingcomputer-executable instructions, or software, for implementing methodsor steps of methods according to the present disclosure. Examples ofsuch media include CD-ROMs, disks, hard drives, flash memory, solidstate memory, etc. As used herein, storage, or memory, devices and mediahaving computer-executable instructions, as well as the various methodsdisclosed herein, are considered to be within the scope of subjectmatter deemed patentable in accordance with Section 101 of Title 35 ofthe United States Code.

FIGS. 1A and 1B show two possible outcomes of printing four colour inkdots (of cyan, magenta, yellow and black) on a small portion of anoutput medium, which an FM screening process has determined should bemarked with all four dots.

Typically, an FM screening process adds a random factor to the selectionof the site for at least one of the dots to be printed in each colour,so that the same intensity of colour components for an image can berepresented by differing distributions of dots, any of whichdistributions achieves the number density of dots to present therequired intensity to the eye.

In scenario A, corresponding to FIG. 1A, the dots have been randomlyplaced so that the cyan dot 1, the magenta dot 2, the yellow dot 4 andthe black, key dot 6 all fall adjacent to each other so as to define a2×2 block of concatenated dots. In scenario B, corresponding to FIG. 1B,all four dots have been randomly placed on the same grid so that theyall fall on top of each other to form a composite dot 8. Although thecomposite dot 8 is darker than the dots 1, 2, 4 and 6, it will filterless light than the four adjacent dots, due to the way in which inkfilters light. Consequently, the four dots in scenario A will appeardarker than the four dots forming the composite dot 8 of scenario B,despite the fact that the same total amount of ink has been applied ineach scenario. It is this random intersection, or not, of dots which isone of the causes of noise when using more than one colour with FMscreening techniques.

FIG. 2 shows a screen capture of a very small part of an imageconstituted by cyan and magenta component colours, applied as dots usingan FM screening process. The areas shown in the Figure use a respectiveshade of grey to indicate each of the areas where there is no overlapbetween any dots, for example as shown at 10, magenta dots, for exampledenoted by 12, cyan dots, for example as shown at 14, and overlappingcyan and magenta dots which are indicated by the darkest shade of grey,for example as shown at 16. Although very similar shades are used forthe cyan dots and the overlapping regions, the Figure does clearly showthat inconsistencies in the extent to which the dots overlap can lead todark areas, such as the area 18, in which a high proportion of the dotsare interleaved, and light areas, such as the area 20, in which a largerproportion of the dots overlap each other. In this example, thevariation is caused by the two dot structures having the same“frequency” and “size”, in this case the same “swirl frequency” and“swirl width”.

FIG. 3 illustrates how cyan and magenta dots can be laid down by atechnique within the scope of the present disclosure. The dots are shownplaced on a grid of 12×12 pixels, each corresponding to a respectivepixel in an output medium. In the Figure, the white pixels are pixels onwhich no drop has been deposited, pixels to which just magenta or cyanink has been applied are shown in the same shade of grey, whilst pixelsto which both inks have been applied are shown as a darker shade ofgrey. Thus, for example, no ink has been applied to the pixel 22,whereas the 2×2 block of pixels 24 have had both types of ink applied,the pixel 26 is magenta and reference numeral 28 denotes a cyan pixel.

Each magenta pixel is part of a respective magenta dot constituted by ablock of three rows and two columns of pixels, whilst each cyan pixel ispart of a dot constituted by two rows and three columns of neighbouringpixels. Each dot lies in a respective, correspondingly sized site ofpossible pixel positions for that dot, each site only ever accommodatingthe pixels from one associated colour of dot, although dots of the othercolour can encroach on an occupied site.

In FIG. 4, thicker lines have been used to denote the arrangement ofpossible sites for the cyan dots. Those possible sites form a 6×4 arraythat tiles the grid area shown in FIG. 3, each site in the array beingof two rows and three columns of pixels (i.e. of a size corresponding tothat of the cyan dots). In this particular example, some of the sites,29-33 contain no marked pixels at all, whilst eight sites, 34-41,contain cyan dots. Each of the sites 34-41, however, can only containpixels from a respective one of the cyan dots.

In FIG. 5, the thicker black lines (for example line 44) highlight agrid of possible sites for the magenta dots. As can be seen from theFigure, each possible site is of a corresponding size to a magenta dot(i.e. three rows by two columns of pixels), and the possible sites forthe magenta dots tile the area in an array of four rows by six columns.Some of the possible sites, for example 46-48, contain no pixels ofeither colour, whilst the magenta dots are situated at sites 49-55.

The allowable locations for the dots of each colour are constrained bythe possible sites for that colour of dot. Thus each cyan dot can onlybe placed in a single respective site for the cyan dots, whilst eachmagenta dot must lie in a single respective possible site for themagenta dots. This means that a dot cannot overlap a number of possiblesites for dots of that colour, and that a dot cannot encroach upon asite for dots of that colour which is already occupied by another dot ofthe same colour. However, a possible site for dots of one colour doesoverlap at least two possible sites for dots of the other colour.Consequently, although there is no overlap of dots of the same colour,dots of different colour can, and frequently do, overlap. This is thecase with the sites 37 and 51 which gave rise to the overlapping block24. Similarly, sites 40 and 50 overlap and, since those sites areoccupied, also accommodate a 2×2 block 56 of pixels formed byoverlapping magenta and cyan dots.

Although a dot of one colour can encroach on a dot of another colour, amagenta and cyan dot cannot completely cover each other because of theirdiffering dimensions. Furthermore, the constraints placed on thepositions of the dots by the possible sites are such that anyopportunities for a dot of one colour to abut an adjacent dot of theother colour are limited. Thus, although the magenta dot at site 49abuts the adjacent cyan dot at site 34, the next available site, site30, for a cyan dot is spaced from the open side of the magenta dot by acolumn of two pixels (58 and 60 in FIG. 4). This arises from the factthat the spacings between the possible sites for the different coloursof dot cannot be in phase with each other since the length/height ofeither possible site is not a multiple of the length/height of the othertype of possible site.

A single dot of one colour will be able randomly to overlap with one,two, three or four pixels of a dot of another colour (i.e. only amaximum of 66% of the other dot).

A screening process according to the present disclosure can thus achievea more consistent extent of overlap/interleaving of dots of thedifferent colours for given shades of output colour.

It will also be appreciated from FIGS. 3-5 that many overlappingportions of two dots span at least one of those dots. For example, theoverlapping portion 56 spans the cyan dot in a possible site 40 from topto bottom. If there is any mis-registration in the application of themagenta or cyan dots which lowers the position of the magenta dot atsite 50 relative to the cyan dot at site 40, this will reduce the amountof magenta dot protruding beyond the top of the cyan dot, but this willbe countered by the area of the magenta dot which would then protrudebeyond the bottom of the cyan dot.

A process according to the present disclosure thus increases thetolerance of the print to mis-registration of dots of different colours(which would otherwise cause colour drift or errors).

In the above example, any site at which a dot of the associated colouris located is fully populated by that dot, so that all of the pixelswithin the site are marked by the colour for that dot. Consequently, themagenta and cyan dots are the same dimensions as their respective sites,and each dot extends to the periphery of its respective site.

However, there are circumstances where it may be desirable to use areduced size of dot (relative to the size of the site) to avoid dotgain, as discussed in WO 2011/030101. In the present case, this can beachieved by consistently reducing the size of the dots of each colour.For example, all the cyan dots can be reduced by omitting their firstcolumn of pixels (i.e. the two pixels on the left-hand side of eachdot), whilst a corresponding reduction can be achieved by omitting thefirst row (i.e. the top two pixels) of each magenta dot.

This means that both types of dot will then be of a size of 2×2 pixels.However, the cyan dots will only occupy the right-hand two-thirds oftheir associated sites, whilst the magenta dots will only occupy thelower two-thirds of their associated sites. This situation isillustrated in FIG. 6, which is a grid similar to that shown in FIGS.3-5, but in which the size of the dots has been reduced (and thelocations of the dots differ from those of the dots shown in FIGS. 3-5).Although both colours of dots are now the same size, their sites are ofthe same dimension as before. This, coupled with the fact that the dotsof each colour can only occupy the same part of the respective possiblesites, means that most of the dots of one colour still cannot becompletely covered by dots of the other colour.

As with FIGS. 3-5, the pixels which are marked with magenta or cyan inkonly are represented by the same shade of grey, whilst the darker shadeof grey represents pixels which have been marked with ink of bothcolours. In order to illustrate the relationship between the dots ofdifferent colours, three possible sites, 62, 64, 66 for the cyan dotsare shown in FIG. 6, whilst reference numerals 68 and 70 denote twopossible sites for the magenta dots. Each of the sites 62, 64, 66, 68and 70 contains a respective one of the dots for the set associated withthat site. Thus, the sites 62, 64 and 66 each contain a respective cyandot of 2×2 pixels, whilst the sites 68 and 70 each contain a respectivemagenta dot, also of 2×2 pixels. The site 72 also accommodates arespective magenta dot.

The cyan dot in the site 62 only occupies the middle and right-handcolumn of pixels in that site, whilst the magenta dot in the site 70only occupies the bottom and middle row of pixels of that site, so thatthe two dots cannot fully overlap, the two pixels 74 being the only areawhere overlap occurs. Similarly, the magenta dot in the possible site 68only occupies the bottom two rows of that site, whilst the cyan dot inthe site 66 only occupies the middle and right-hand columns of pixels ofthat site and the cyan dot in the site 64 similarly only occupies themiddle and right-hand columns of that site. Thus the two pixels 76 aremarked cyan ink only, and the two pixels 80 are marked with ink of bothcolours. The only areas in which a cyan dot can completely cover amagenta dot are in the bottom right-hand corner of each of an array of6×6 pixel cells. The 2×2 pixel blocks to which this applies in FIG. 6are indicated by reference numerals 82, 84, 86 and 88.

Each of those cells can be considered to be a stochastic rosette cell,and the repetition occurs at a high frequency that does not cause theeye any problems. FIG. 7 shows a large area to which the reduced sizeddots have been applied. As can be seen, even with the reduced size dots,a far better consistency of overlap/interleaving for colour images isachieved than with the prior art screening method, which gives rise tothe variations shown at 18 and 20 in FIG. 2.

The way in which the present screening process converts a multi-bitimage input into a 1-bit output array for the dots mentioned above issimilar to the known screening processes, although the differentdimensions of the two different types of possible sites are taken intoaccount in the initial scaling of the multi-bit input array.

FIG. 8 shows a portion 200 of a multiple-bit image data array which hasbeen divided up into a number of subsets, 210-226. Each subset containssixteen 8-bit pixel values each represented by a respective number. Eachpixel value represents the desired intensity for a component colour of acorresponding pixel in the output image. Each element of the array maybe a word of a sufficient number of bits to convey intensity informationfor all the component colours, or the multi-bit array may apply to justone of the component colours. In either case, the division of theindividual values into subsets will correspond to the way in which thepixels are mapped onto the dot sites in the final image. Thus, forexample, a portion of the input data array for a 2400 dpi image willhave 2400 rows and 2400 columns of multiple-bit pixel values per squareinch. If these values are for mapping onto cyan dots, those values willbe allocated into 800×1200 subsets per square inch for cyan. If thevalues are for magenta dots, they will be allocated into 1200×800subsets. If the values are for black intensity levels, they will then bemapped into 600×600 subsets per square inch (since the black possiblesites are of 4×4 pixels).

To obtain the second array, the first intermediate array is processedusing an error diffusion algorithm which is applied to each averagepixel value (i.e. average value for each of the subsets of the inputarray) of the first intermediate array in turn, starting at the top leftaverage pixel value, working along the first row of average pixel valuesto the top right average pixel value then proceeding to the right mostaverage pixel value of the second row, and so on. For the first averagepixel value, the algorithm will typically add a random number torandomise the selection of the first dot (i.e. ‘on’ value) on the secondoutput array. The sum of the random number and the average is subjectedto a threshold test. For example, that threshold may be 127 (about halfof the 255 maximum value that an 8-bit word can represent). If the sumis greater than the threshold then the first entry in the output arrayis set to “on”.

This is what has happened in the present example, as is indicated at 510in FIG. 9.

For each average pixel value in turn, unless the corresponding pixelvalue of the second intermediate array has already been set to “on”, theerror diffusion algorithm determines from the average pixel value andany diffused error whether the corresponding pixel value of the secondintermediate array should be set to “on”. If it is determined that thecorresponding pixel value should be set to “on”, a first series of testsare applied based on the average pixel value. If the results of thetests are all negative, the corresponding pixel value is set to “on”,otherwise the corresponding pixel value remains set to “off”. Similarly,if it is determined by the error diffusion algorithm that thecorresponding pixel value should be set to “off”, a second series oftests are applied based on the average pixel value. If the results ofthe tests are all negative, the corresponding pixel value remains set to“off”. If any of the results of the tests is positive, the correspondingpixel value is set to “on”.

FIG. 9 represents a portion 500 of the second intermediate array, “on”pixel values being represented by shaded squares and “off” pixel valuesbeing represented by unshaded squares. The square 510 represents thepixel value that corresponds to the average pixel value calculated fromthe sub-array 210 of FIG. 8. The squares 512, 514, 516, 518, 520, 522,524 and 526 correspond to the average pixel values calculated from thesub-arrays 212, 214, 216, 218, 220, 222, 224 and 226 of FIG. 8.

The average values set out in the first array are also subject to thefollowing series of tests.

(a) If the average pixel value is less than or equal to 50%, i.e. 127for 8-bit image data, would setting the pixel value of the secondintermediate array to “on” cause a row of more than 4 horizontallyadjacent “on” values or a column of more than 4 vertically adjacent “on”values?

(b) If the average pixel value is greater than 50% and less than orequal to 62.5%, would setting the pixel value of the second intermediatearray to “on” cause a row of more than int (4+((average pixelvalue−50)/12.5)*7) horizontally adjacent “on” values or a column of morethan int (4+((average pixel value−50)/12.5)*7) vertically adjacent “on”values?

(c) If the average pixel value is less than or equal to 28.125%, wouldsetting the pixel value of the second intermediate array to “on” cause achain of 3 horizontally and/or vertically adjacent “on” values?

(d) If the average pixel value is less than or equal to 50%, wouldsetting the pixel value to “on” cause a block of 2*2 horizontally andvertically adjacent “on” values?

(e) If the average pixel value is greater than 50% and less than 56.25%,would setting the pixel value to “on” cause a block of 2*3 or 3*2horizontally and vertically adjacent “on” values?

(f) If the average pixel value is greater than 56.25% and less than orequal to 62.5%, would setting the pixel value to “on” cause a block of3*3 or 2*4 or 4*2 horizontally and vertically adjacent “on” values?

(g) If the average pixel value is greater than 62.5% and less than orequal to 68.75%, would setting the pixel value to “on” cause a block of3*4 or 4*3 or 2*6 or 6*2 horizontally and vertically adjacent “on”values?

(h) If the average pixel value is greater than 68.75% and less than orequal to 75%, would setting the pixel value to “on” cause a block of 4*4or 3*5 or 5*3 or 2*7 or 7*2 horizontally and vertically adjacent “on”values?

(i) If the average pixel value is greater than 75% and less than orequal to 82.5%, would setting the pixel value to “on” cause a block of4*5 or 5*4 or 3*7 or 7*3 or 2*10 or 10*2 horizontally and verticallyadjacent “on” values?

In this instance, because the average pixel value is the first pixelvalue of the first intermediate array to be processed, all of the pixelvalues of the second intermediate array are “off” by default and theresults of all of the tests are negative. The pixel value 510 istherefore kept “on”.

Had the error diffusion algorithm determined that the correspondingpixel value should remain “off”, a corresponding series of tests wouldhave been carried out. The test corresponding to test (a), for example,is “If the average pixel value is greater than or equal to 50%, wouldsetting the pixel value of the second intermediate array to ‘off’ causea row of more than 4 horizontally adjacent ‘off’ values or a column ofmore than 4 vertically adjacent ‘off’ values?” The test corresponding totest (i) is “If the average pixel value is less than 25% and greaterthan or equal to 17.5%, would setting the pixel value to ‘off’ cause ablock of 4*5 or 5*4 or 3*7 or 7*3 or 2*10 or 10*2 horizontally andvertically adjacent ‘off’ values?”

By setting the pixel value of the second intermediate array to “on”, thecorresponding average pixel value of 134 of the first intermediate arrayhas effectively been represented by a pixel value of 255, the pixelvalues of the second array being constrained to be either 0 or 255,which correspond to “off” and “on” respectively. An error value of255−134=121 is therefore generated and subtracted from the average pixelvalue of the sub-array 212.

With the exception of portions of the image of which the densities ofdots are low, and only then if it was determined in operation 140 thatsingle dots are to be used in portions of the image of which thedensities of dots are low, a dot must always form a pair with ahorizontally or vertically adjacent dot. This means that a horizontallyor vertically adjacent pixel value of the second intermediate array mustalso be set to “on”, so that the image contains a pair of horizontallyor vertically adjacent dots. The horizontally or vertically adjacentpixel value is chosen at random. In this instance, the verticallyadjacent pixel value 520 is chosen and set to “on”.

Processing of the first intermediate array moves to the average pixelvalue calculated from the sub-array 212. The average pixel value is 141to the nearest whole number. The result of subtracting the error of 121diffused from the previous average pixel value from the average pixelvalue 141 is 20. As this is less than 127, the error diffusion algorithmdetermines that the pixel value 512 of the second intermediate arrayshould remain “off”.

The results of the corresponding series of tests are all negative so thepixel value 512 remains “off”. Again, with the exception of portions ofthe image of which the densities of dots are high, and only then if itwas determined in operation 140 that single dots are to be used inportions of the image of which the densities of dots are low, and thatalso therefore single non-dots are to be used in portions of the imageof which the densities of dots are high, a non-dot must always form apair with a horizontally or vertically adjacent non-dot. This means thata pixel value horizontally or vertically adjacent to the pixel value 512must also remain “off”. In this instance the horizontally adjacent pixelvalue 514 is chosen.

By maintaining the pixel value 512 “off”, the result of thecorresponding average pixel value less the error diffused from theprevious pixel value, that is 20, has effectively been represented by apixel value of 0. An error value of 0−20=−20 is therefore generated andsubtracted from the average pixel value of the sub-array 214.

Processing of the first intermediate array moves to the average pixelvalue calculated from the sub-array 214. The average pixel value is 137to the nearest whole number. The result of subtracting the error of −20diffused from the previous average pixel value from the average pixelvalue 137 is 157. Although this is greater than 127, the pixel value 514has already been determined to remain “off”, so as to form a pair withthe “off” pixel value 512. An error value of 0−157=−157 is thereforegenerated and subtracted from the average pixel value of the sub-array516.

Processing moves to the average pixel value calculated from thesub-array 216. The average pixel value is 139 to the nearest wholenumber. The result of subtracting the error of −157 diffused from theprevious average pixel value from the average pixel value 139 is 296. Asthis is greater than 127, the error diffusion algorithm determines thatthe pixel value 516 of the second intermediate array should be set to“on”.

The results of the series of tests are all negative so the pixel value516 is set to “on”. The horizontally adjacent pixel value 518 is chosenas the adjacent pixel that is to be set to “on” so that the pixel values516 and 518 form a pair. An error value of 255−296=−41 is generated andsubtracted from the average pixel value of the sub-array 218.

Processing moves to the average pixel value calculated from thesub-array 218. The average pixel value is 143. The result of subtractingthe error of −41 diffused from the previous average pixel value from theaverage pixel value 143 is 184. As the pixel value 518 has already beendetermined to be set to “on”, the tests are not carried out and an errorvalue of 255−184=71 is generated and subtracted from the average pixelvalue of the sub-array 220.

Processing moves to the average pixel value calculated from thesub-array 220. The average pixel value is 137 to the nearest wholenumber. The result of subtracting the error of 71 diffused from theprevious average pixel value from the average pixel value 137 is 66.Although this is less than 127, the pixel value 520 has already been setto “on”, so as to form a pair with the pixel value 510. An error valueof 255−66=189 is therefore generated and subtracted from the averagepixel value of the sub-array 222.

Processing moves to the average pixel value calculated from thesub-array 222. The average pixel value is 127 to the nearest wholenumber. The result of subtracting the error of 189 diffused from theprevious average pixel value from the average pixel value 127 is −62. Asthis is less than 127, the error diffusion algorithm determines that thepixel value 522 should remain “off”.

The results of the corresponding series of tests are all negative so thepixel value 522 remains “off”. The horizontally adjacent pixel value 524is chosen as the adjacent pixel that is to remain “off” so that thepixel values 522 and 524 form a pair. An error value of 0−(−62)=62 isgenerated and subtracted from the average pixel value of the sub-array224.

Processing moves to the average pixel value calculated from thesub-array 224. The average pixel value is 133 to the nearest wholenumber. The result of subtracting the error of 62 diffused from theprevious average pixel value from the pixel value 133 is 71. As thepixel value 524 has already been determined to remain “off”, the testsare not carried out and an error value of 0−71=−71 is generated andsubtracted from the average pixel value of the sub-array 226.

Processing moves to the average pixel value calculated from thesub-array 226. The average pixel value is 131 to the nearest wholenumber. The result of subtracting the error of −71 diffused from theprevious average pixel value from the pixel value 131 is 202. As this isgreater than 127, the error diffusion algorithm determines that thepixel value 526 should be set to “on”.

The series of tests set out above is carried out, the results all beingnegative. The pixel value 526 is therefore set to “on”. There is no needto set a horizontally or vertically adjacent pixel value to “on” becausethe pixel value 516 is vertically adjacent to the pixel value 526 and isset to “on”, so that the pixel values 516 and 526 form a pair.

As can be seen from this simple example, the specimen multiple-bit imagedata of FIG. 8 have an average value of 136 to the nearest whole number.This corresponds to approximately 53%, the maximum possible value of themultiple-bit data being 255. Accordingly, five of the nine pixel valuesof the second intermediate array that correspond to the nine averagepixel values of the first intermediate array calculated from the ninesub-arrays shown in FIG. 8 are “on”. It is to be noted that theoperation of a computer program according to the present disclosure issuch that, as shown in FIG. 9, isolated pairs of diagonally adjacent“on” pixel values do not occur, pairs of diagonally adjacent “on” pixelvalues such as 510 and 518, and 518 and 526, either forming a tripletwith a third “on” pixel value such as 520 or 516, or forming aquadruplet, each of the pair of diagonally adjacent “on” pixel valuesforming a pair with a (shared) horizontally or vertically adjacent “on”pixel value.

This screening process is substantially as described in PCTInternational Application Publication No. WO 2011/030102 and U.S. PatentApplication Publication No. 2012/0218607, the contents of which arehereby incorporated by reference.

The second intermediate array shown at FIG. 9 is one of two such arrays,each for a respective colour of dot. Each of the squares in the Figurecorresponds to a respective possible site for a dot, the shaded squaresdenoting sites to be occupied by the corresponding dots.

For the cyan dots, the second intermediate array is of a size 800×1200elements per square inch, whilst the size of the second array formagenta dots is 1200×800 elements per square inch, so that when eachelement is used to instruct the marking of a respective possible site,the original 2400×2400 scale is preserved in the final image.

The process will also apply yellow dots to the substrate to give a CMYKcolour image, but the process of screening the data for the yellow dotsand the selection of the dimensions of those dots and their possiblesites can be in accordance with conventional FM screening techniquessince yellow is a relatively light colour and the variations (if any) ininterleaving and overlap of the yellow dots with the other dots will notsignificantly impair image quality.

FIG. 10 shows printing apparatus comprising a computer 600 loaded with aprogram for performing a screening process according to the presentdisclosure, and a printer such as a CTP printer 620 for printing theresultant 1-bit image data onto a suitable carrier.

The disclosure set forth above encompasses multiple distinct inventionswith independent utility. While each of these inventions has beendisclosed in its preferred form or method, the specific alternatives,embodiments, and/or methods thereof as disclosed and illustrated hereinare not to be considered in a limiting sense, as numerous variations arepossible. The present disclosure includes all novel and non-obviouscombinations and subcombinations of the various elements, features,functions, properties, methods, and/or steps disclosed herein.Similarly, where any disclosure above or claim below recites “a” or “afirst” element, step of a method, or the equivalent thereof, suchdisclosure or claim should be understood to include incorporation of oneor more such elements or steps, neither requiring nor excluding two ormore such elements or steps.

It is believed that the following claims particularly point out certaincombinations and subcombinations that are directed to one of thedisclosed inventions and are novel and non-obvious. Inventions embodiedin other combinations and subcombinations of features, functions,elements, properties, methods, and/or steps may be claimed throughamendment of the present claims or presentation of new claims in this ora related application. Such amended or new claims, whether they aredirected to a different invention or directed to the same invention,whether different, broader, narrower, or equal in scope to the originalclaims, also are regarded as within the subject matter of the inventionsof the present disclosure.

The invention claimed is:
 1. A method of printing a colour image havingmore than one colour and derived from multiple-bit image data, themethod comprising the steps of: (a) receiving multiple-bit image datacomprising multiple-bit pixel values; (b) deriving from saidmultiple-bit pixel values 1-bit image data comprising a first set and asecond set of 1-bit image data comprising “on” and “off” pixel values,each set corresponding to a respective component colour of the colourimage; and (c) printing from the 1-bit image data the colour imagecomprising corresponding first and second sets of dots, the dots of eachset being of a respective one of the component colours, each of all ofthe dots of one of the colours of dots being constrained to be within arespective site selected from a first set of predetermined possiblesites, and each of all of the dots of the other colour of dots beingconstrained to be within a respective site selected from a second set ofpredetermined possible sites, wherein the possible sites of the firstand second sets of possible sites overlap so that individual dots of onecolour of dots can overlap dots of the other colour of dots, and whereinthe possible sites of one set are of different dimensions from the sitesof the other set so that individual dots of one colour cannot fullycover individual dots of the other colour, wherein the possible siteshave the same shape and size, the possible sites having differentdimensions by virtue of the sites of the first set of possible siteshaving a different orientation from the sites of the second set ofpossible sites.
 2. The method of claim 1, in which each set of the firstand second sets of possible sites is tessellated, and wherein the firstand second sets of possible sites cover the same area of the image. 3.The method of claim 1, wherein the possible sites of the first set ofpossible sites are all of the same shape and dimensions as each other,and the possible sites of the second set of possible sites are also allof the same shape and dimensions as each other.
 4. The method of claim3, in which all the possible sites of the first set of possible sitesare each of a size of M1×N1 pixels, whilst all of those of the secondset of possible sites are of a size M2×N2 pixels, wherein M1 does notequal M2 and N1 does not equal N2.
 5. The method of claim 4, in whicheach possible site of the first set of possible sites is two units highby three units wide (2×3), whilst each possible site of the second setof possible sites is three units high by two units wide (3×2).
 6. Themethod of claim 5, in which each unit corresponds to a respective pixelin the colour image so that the sites of the first set of possible sitesare each defined by two rows and three columns of pixels, the possiblesites of the second set of possible sites each being defined by threerows and two columns of pixels.
 7. The method of claim 3, in which awidth of each site in the first set of possible sites is not a multipleor a factor of a width of each site in the second set of possible sites.8. The method of claim 3, in which none of the individual possible sitesof the first set of possible sites fully covers any one of theindividual possible sites of the second set of possible sites, and viceversa.
 9. The method of claim 1, in which each dot of the first andsecond sets of dots is coextensive with its respective site, so thateach dot has the same size, shape, and orientation as its respectivesite.
 10. The method of claim 1, in which each dot of the first andsecond sets of dots is smaller than its respective site, each dot of agiven set of the first and second sets of dots being constrained to bewithin its respective site, the dots being so arranged that asignificant number of individual dots from the first set cannotcompletely cover individual dots from the second set.
 11. The method ofclaim 10, in which the dots of the first and second sets of dots occupycorresponding portions of their respective sites.
 12. The method ofclaim 11, in which the dots of the first and second sets of dots arereduced from a full size of 3×2and 2×3 units by omitting a first row ofall of the 3×2 dots and a first column of all of the 2×3 dots.
 13. Themethod of claim 1, in which pixel data for the first set of dots isassociated with possible sites of 2×3units in size and is for one of thecyan component or the magenta component of the colour image, and inwhich pixel data for the second set of dots is associated with possiblesites of 3×2 units in size and is for the other of the cyan component orthe magenta component of the colour image.
 14. The method of claim 13,in which the 1-bit image data includes data for a third set of dots, andassociated possible sites, corresponding to a black component, the sizeof each site of the third set being 4×4 units.
 15. The method of claim1, in which a height of each site in one of the first and second sets ofpossible sites is not a multiple or a factor of a height of each site inthe other of the first and second sets of possible sites.
 16. The methodof claim 1, in which a width of each site of all of the first set ofpossible sites is not a multiple or a factor of a width of each site inall of the second set of possible sites, and wherein a height of eachsite in all of one of the first and second sets of possible sites is nota multiple or a factor of a height of each site in all of the other ofthe first and second sets of possible sites.
 17. The method of claim 1,wherein the step (b) comprises a stochastic screening method.
 18. Aprinting apparatus programmed or configured to perform the method ofclaim 1 to print the colour image.
 19. A method of generating 1-bitimage data for a colour image having more than one colour, the 1-bitimage data comprising at least two sets of pixel values, each setcorresponding to a respective component colour of the colour image, themethod comprising the steps of: (a) receiving multiple-bit image datacomprising multiple-bit pixel values; and (b) using electronic dataprocessing apparatus, electronically deriving from said multiple-bitpixel values a first set and a second set of 1-bit image data comprising“on” and “off” pixel values which produce when printed a first andsecond set of dots, the dots of each set being of a respective one ofthe component colours, each of all of the dots of one of the colours ofdots being constrained to be within a respective site selected from afirst set of predetermined possible sites, and each of all of the dotsof the other colour of dots being constrained to be within a respectivesite selected from a second set of predetermined possible sites, whereinthe possible sites of the first and second sets of possible sitesoverlap so that individual dots of one colour of dots can overlap dotsof the other colour of dots, and wherein the possible sites of one setare of different dimensions from the sites of the other set so thatindividual dots of one colour cannot fully cover individual dots of theother colour, wherein the possible sites have the same shape and size,the possible sites having different dimensions by virtue of the sites ofthe first set of possible sites having a different orientation from thesites of the second set of possible sites.
 20. Electronic dataprocessing apparatus programmed or arranged to perform the method ofclaim 19.